What are the minimum propping period of beams and slabs of various spans?

It depends. Concrete gains strength at a non-linear rate (Initially high, later low) and at any stage of time, as long as its strength > strength requirements, you don't need any propping. Mostly, during construction, the loads acting on the beams/slabs are 1) Self weight 2) Construction load (or) 3) Weight of upper floor (from the props !) but this is a separate load case